In this study, elliptical concave impaction plate was suggested for lowering cut-off size and therefore enhancing collection efficiency of the inertial impactor. Statistical Lagrangian Particle Tracking (SLPT) model was employed for calculating impactor collection efficiency and validated by comparing with the experimental data of Tsai, C.J., Cheng, Y.H. ((1995). Solid particle collection characteristics on impaction surfaces of different designs. Aerosol Science Technology, 23, 96-106), for three different shapes of impaction plates. Then, the effect of the ratio of major axis length (A) to minor axis length (B) for determining the curvature of elliptical concave impaction plate, on impactor collection efficiency was numerically investigated using the SLPT model, with nozzle Reynolds numbers ranging from 1440 to 2600. It was found that there existed an optimum range of the A/B ratio for minimizing the cut-off size, i.e. the A/B ratio ranged between 3.2 and 4.2 for the PM10 inertial impactor, or between 3.2 and 3.5 for the PM2.5 inertial impactor. When the elliptical concave impaction plates with the A/B ratio of 4.0 and 3.5 were applied to the MST indoor air sampling impactor having PM10 and PM2.5 stages, the cut-off size was predicted to decrease from 10 to 6.5 mu m and from 2.5 to 1.6 mu m, respectively, while the impactor collection efficiency curves became less steep. (C) 2011 Elsevier Ltd. All rights reserved.